This invention relates to an apparatus for controlling an internal combustion engine of the spark ignition type including a spark-timing control means for controlling the timing of sparks supplied to the engine and, more particularly, to an engine control apparatus selectively using one of two tables to calculate a value corresponding to a setting of the spark-timing control means, the first table describing a desired relationship between the engine condition and the spark-timing control means for the engine being operating on low octane fuel, the second table describing a desired relationship between the engine condition and the spark timing control means for the engine being operating on high octane fuel.
In the field of internal combustion engines, detonation or knock is an undesirable event. In the past, it has been avoided by limiting the compression ratio or by providing gasoline with sufficient octane. Normally, the available lower octane fuel, which is referred to as regular leaded gasoline, has an octane number of about 83 and the available higher octane fuel, which is referred to as high-octane leaded gasoline, has an octane number of about 93. It is the current practice to increase the engine thermal efficiency by advancing the timing of sparks supplied to the engine as greater as possible to operate the engine in a condition close to a knock condition. For this purpose, high-octane leaded gasoline is useful to provide a higher anti-knock property.
If regular leaded gasoline is used for an engine designed to operate on high-octane fuel, however, the engine cannot provide sufficient engine output nor operate in a smooth manner since knocking occurs frequently. In order to avoid the problem, attempts have been made to control the spark timing with the use of selected one of two tables, the first table describing a desired relationship between the engine condition and the spark timing for the engine being operating on low octane fuel, the second table describing a desired relationship between the engine condition and the spark timing for the engine being operating on high octane fuel. For example, Kokai Jananese Pat. No. 60-212673 discloses such an engine control apparatus. The engine control apparatus includes a spark timing control unit and calculates a value corresponding to a setting of the spark timing control unit from the first table after the engine starts. When the engine condition is within a specified range of values of engine speed and engine load, the engine control apparatus performs advance control to advance the spark timing at a predetermined rate. The used table is changed from the first table to the second table when the spark timing is advanced over a first reference value during the advance control. The engine control apparatus performs knock control to retard the spark timing at a predetermined rate when knocking occurs during the course of the spark timing control with the use of the second table. The used table is changed from the second table to the first table when the spark timing is retarded over a second reference value during the knock control.
Since the conventional engine control apparatus cancels the value by which the spark timing is advanced relative to the calculated value during the advance control when the engine condition is out of the specified range and advances the spark timing at the predetermined rate from a calculated value during the successive advance control, however, it cannot make a smooth change from the first table to the second table even though the engine is operating on high octane fuel when the engine condition remains in a short time within the specified range. For this reason, the conventional engine control apparatus fails to derive the advantageous properties of the high octane fuel to a sufficient extent.